Broadly speaking, heat engine driven heat pump systems are well known and have been refined for internal space conditioning use as shown in U.S. Pat. No. 4,991,450. Because heat engines, such as natural gas driven internal combustion engines, provide excess and otherwise unused heat in the motive process, subsystems have been developed which recapture otherwise waste heat which is circulating in the engine coolant. U.S. Pat. Nos. 5,003,788, 5,020,320, 5,029,449, and 5,099,651 are further examples of this type of subsystem.
Further improvements in these subsystems provide advantages in waste heat recovery to the application of such heat to the occupants of the space and in some instances for other purposes, such as domestic water heating.
Traditionally heat pumps such as electric motor-driven heat pumps do not have sufficient excess available heat for use in such subsystems. Thus, waste heat recovery components are not included in electric motor-driven heat pumps.
Further refinements and improvements in waste heat recovery subsystems are important since they increase the overall coefficient of performance (COP) of the heat pump system as well as providing overall operational economies by reducing the amount of externally supplied auxiliary heat and increase the comfort of the delivered air. Waste heat recovery is possible, however, only if necessary coolant flow to the engine is provided to receive the waste heat, and the necessary coolant flow to heat recovery components is maintained when and where needed to deliver waste heat to desired loads.
It has been found through testing that pockets of air and vapor may become trapped or accumulate in various locations in the engine coolant and heat recovery subsystem. Trapped air gases and vapors can result from initial filling, or can accumulate during system operation from entrainment in the circulating fluid. Regardless, the resulting pockets of trapped air and vapor inhibit and interfere with necessary coolant flow for engine cooling and waste heat recovery, preventing efficient waste heat recovery and use.
For example, trapped air and vapor pockets can prevent adequate purging upon initial coolant charging of the subsystem, resulting in subsystem overheating during operation. To overcome this problem, the initial purge process has become more involved and more time-consuming than desirable, for example, requiring rapid flushing and repeated manual venting, adding to overall system operating costs. However, in some systems, complete purging of gases and vapors from the coolant is not possible, and entrained gases and vapors continue to accumulate during operation. Further, where present, trapped air and vapor pockets can cause increased operating temperatures, resulting in long-term coolant degradation and reduced coolant life, impacting overall heat pump system reliability. Ultimately, trapped air and vapor pockets in high points of subsystem lines and components, often remote and elevated relative to the heat engine, can prevent the pump from providing adequate coolant flow, impacting not only the delivery of waste heat, but return coolant flow to the engine. In such cases, engine driven pumps have been found to have insufficient static pressure capabilities to overcome air and vapor pockets, for example, pockets trapped or accumulated in the coolant line to the indoor coil, especially where the engine is operating at low speeds.
While this invention is described herein in association with a gas fueled internal combustion engine, broader applications to other "heat" engines, such as turbines, may be possible. The coolant fluid employed in the subsystem may be one of various conventional types, such as an ethylene glycol-water mixture or a propylene glycol-water mixture.
Accordingly, the need exists for improved engine coolant circulation systems to improve coolant circulation, waste heat recovery, and operating efficiencies of such engine-driven heat pump systems.